1. Field of the Invention
The invention relates to a process for preparing solutions, of improved color, of polyhydroxy-fatty acid amide which has been obtained by reaction of N-alkylpolyhydroxyamine and fatty acid alkyl ester. The invention furthermore relates to the use of these solutions.
2. Description of the Prior Art
Polyhydroxy-fatty acidamides are valuable surface-active compounds which can be employed for many uses. They can thus be employed, for example, as such or as a mixture with anionic, cationic and/or nonionic surfactants as cleaning agents, detergents, textile treatment agents and the like, and in particular in the form of solid products (for example as powders, grains or granules), solutions, dispersions, emulsions, pastes and the like. Since polyhydroxy-fatty acid amides are also readily biologically degradable and can be prepared from regenerating raw materials, they have recently acquired relatively great importance.
The polyhydroxy-fatty acid amides in question are as a rule compounds of the formula R--CO--NR'--Z, in which R is a hydrocarbon radical having about 5 to 30 carbon atoms, preferably 8 to 18 carbon atoms, R' is H, alkyl or hydroxyalkyl having up to preferably 8 carbon atoms and Z is a polyhydroxy hydrocarbon radical having at least three OH, which can also be alkoxylated, preferably a sugar alcohol radical. The preferred polyhydroxy-fatty acid amides thus correspond to the following formula ##STR1## in which R.sup.1 is a short-chain alkyl, such as C.sub.1 to C.sub.4 -alkyl, or hydroxyalkyl, such as --CH.sub.2 CH.sub.2 OH, R.sup.2 is a fatty alkyl and n is 3 or 4. The compounds where n=4, which are particularly preferred, are called glycamides, and in the case of glucose as the hexose radical, glucamides. The fatty acid alkyl esters are in general fatty acid C.sub.1 to C.sub.4 -alkyl esters, methyl, ethyl, propyl or isopropyl being preferred. The fatty acid methyl esters are particularly preferred. The fatty acid radical (the acyl group) in general has 6 to 24 carbon atoms, preferably 8 to 18 carbon atoms. It can be saturated or unsaturated (preferably mono- to tri-unsaturated).
Polyhydroxy-fatty acid amides are in general prepared by reaction of an N-alkylpolyhydroxyamine (for example N-alkylglucamine) with a fatty acid alkyl ester in the presence of basic catalysts, the reaction being carried out in the melt (in bulk) or with the aid of solvents. The following reaction equation with N-methylglucamine and lauric acid methyl ester is intended to illustrate this in more detail: ##STR2## It is generally preferable to carry out the reaction in the presence of solvents.
Such preparation processes are described in a number of publications, of which WO 92/06073 and WO 94/10130 may be mentioned here as representative. The N-alkylpolyhydroxyamine and the fatty acid alkyl ester are employed in an essentially equimolar amount. The reaction is carried out at a temperature of up to about 135.degree. C. Basic catalysts which are employed are, preferably, alkali metal hydroxides, alkali metal carbonates and/or C.sub.1 to C.sub.4 alkali metal alkoxides, such as sodium methylate and potassium methylate, and solvents which are employed are C.sub.1 - to C.sub.4 -alcohols, glycerol and/or glycols, such as ethylene glycol and propylene glycol (1,2-propylene glycol and 1,3-propylene glycol). The amount of catalyst is 0.1 to 20 mol %, based on the fatty acid ester. The solvent is employed in an amount such that the resulting solution of polyhydroxy-fatty acid amide is 10 to 80% strength by weight. If the solvent is expediently removed using a vacuum, the polyhydroxy-fatty acid amide is obtained in solid form, for example as a powder. The resulting polyhydroxy-fatty acid amide products, whether in solid form or in the form of solutions, do not have the desired color quality, and rather they are in general pale brown to dark brown in color.
There may also be mentioned as prior art the publication WO 93/09215, in which liquid color-stabilized detergent formulations are described, including, inter alia, those comprising polyhydroxy-fatty acid amides. Protection against a change in color of the fresh formulations is achieved by adding to the formulations 0.001 to 10% by weight, based on the total formulation, of one or more of the following color-stabilizing compounds: sulfites, hydrogen sulfites or pyrosulfites, sulfur dioxide, sulfurous acid, .alpha.-hydroxyalkylsulfonic acids, mercaptoethanol, sodium mercaptoacetate, 2-aminoethanethiol, cysteine, polycysteine, glutathione and formamidinesulfinic acid.